Method and Apparatus for Cooling in Liquefaction Process

ABSTRACT

Methods and apparatus are disclosed for efficient cooling within air liquefaction processes with integrated use of cold recycle from a thermal energy store.

FIELD OF THE INVENTION

The present invention relates to cryogenic energy storage systems, andparticularly to methods for the efficient balancing of the liquefactionprocess with the integrated use of cold recycle from an external sourcesuch as a thermal energy store.

BACKGROUND OF THE INVENTION

Electricity transmission and distribution networks (or grids) mustbalance the generation of electricity with the demand from consumers.This is normally achieved by modulating the generation side (supplyside) by turning power stations on and off, and running some at reducedload. As most existing thermal and nuclear power stations are mostefficient when run continuously at full load, there is an efficiencypenalty in balancing the supply side in this way. The expectedintroduction of significant intermittent renewable generation capacity,such as wind turbines and solar collectors, to the networks will furthercomplicate the balancing of the grids, by creating uncertainty in theavailability of parts of the generation fleet. A means of storing energyduring periods of low demand for later use during periods of highdemand, or during low output from intermittent generators, would be ofmajor benefit in balancing the grid and providing security of supply.

Power storage devices have three phases of operation: charge, store anddischarge. Power storage devices generate power (discharge) on a highlyintermittent basis when there is a shortage of generating capacity onthe transmission and distribution network. This can be signalled to thestorage device operator by a high price for electricity in the localpower market or by a request from the organisation responsible for theoperating of the network for additional capacity. In some countries,such as the United Kingdom, the network operator enters into contractsfor the supply of back-up reserves to the network with operators ofpower plants with rapid start capability. Such contracts can covermonths or even years, but typically the time the power provider will beoperating (generating power) is very short. In addition, a storagedevice can provide an additional service in providing additional loadsat times of oversupply of power to the grid from intermittent renewablegenerators. Wind speeds are often high overnight when demand is low. Thenetwork operator must either arrange for additional demand on thenetwork to utilise the excess supply, through low energy price signalsor specific contracts with consumers, or constrain the supply of powerfrom other stations or the wind farms. In some cases, especially inmarkets where wind generators are subsidised, the network operator willhave to pay the wind farm operators to ‘turn off’ the wind farm. Astorage device offers the network operator a useful additional load thatcan be used to balance the grid in times of excess supply.

For a storage device to be commercially viable the following factors areimportant: capital cost per MW (power capacity), MWh (energy capacity),round trip cycle efficiency and lifetime with respect to the number ofcharge and discharge cycles that can be expected from the initialinvestment. For widespread utility scale applications it is alsoimportant that the storage device is geographically unconstrained—it canbe built anywhere, in particular next to a point of high demand or nextto a source of intermittency or a bottleneck in the transmission anddistribution network.

One such storage device technology is the storage of energy usingcryogen such as liquid air or nitrogen (Cryogenic Energy Storage (CES))which offers a number of advantages in the market place. Broadlyspeaking a CES system would, in the charge phase, utilise low cost orsurplus electricity, at periods of low demand or excess supply fromintermittent renewable generators, to liquefy a working fluid such asair or nitrogen. This is then stored as a cryogenic fluid in a storagetank, and subsequently released to drive a turbine, producingelectricity during the discharge or power recovery phase, at periods ofhigh demand or insufficient supply from intermittent renewablegenerators.

Cryogenic Energy Storage (CES) Systems have several advantages overother technologies in the market place, one of which is their foundingon proven mature processes. Means to liquefy air, necessary in thecharging phase, have existed for more than a century; early systemsutilised a simple Linde cycle in which ambient air is compressed to apressure above critical 38 bar), and progressively cooled to a lowtemperature before experiencing an isenthalpic expansion through anexpansion device such as a Joule-Thomson valve to produce liquid. Bypressurising the air above the critical threshold, the air developsunique characteristics and the potential for producing large amounts ofliquid during expansion. The liquid is drained off and the remainingfraction of cold gaseous air is used to cool the incoming warm processstream. The amount of liquid produced is governed by the required amountof cold vapour and inevitably results in a low specific yield.

An evolution of this process is the Claude cycle (for which the currentstate of the art is shown in FIG. 4); the process is broadly the same asthe Linde cycle however one or more streams 36, 39 are separated fromthe main process stream 31 where they are expanded adiabatically throughturbines 3, 4, resulting in a lower temperature for a given expansionratio than an isenthalpic process and hence efficient cooling. The airexpanded through turbines 3, 4 then rejoins the returning stream 34 andaids the cooling of the high pressure stream 31 via heat exchanger 100.Similar to the Linde cycle the bulk of liquid is formed via expansionthrough an expansion device such as a Joule-Thomson valve 1. The mainimprovement with the Claude process is that power produced by theexpansion turbines 3, 4 directly or indirectly reduces the overall powerconsumption, resulting in greater energy efficiency.

The most efficient modern air liquefaction processes typically use a twoturbine Claude design, and at commercial scale can typically achieve anoptimum specific work figure of around 0.4 kWh/kg. Although highlyefficient this would not enable a CES system to achieve a market entryRound Trip Efficiency figure of 50%, without significant reductions inspecific work.

In order to achieve greater efficiencies the liquefaction process withina fully integrated CES system, such as the one disclosed inW02007-096656A1, utilises cold energy captured in the evaporation of thecryogen during the power recovery phase. This is stored by means of anintegrated thermal store, such as the one detailed in GB 1115336.8, andthen used during the charging phase to provide additional cooling to themain process stream in the liquefaction process. The effective use ofthe cold recovery stream is a prerequisite to achieving an efficientcryogenic energy storage system.

The necessary change in enthalpy that an arbitrary high pressure processstream must undergo to reach the required temperature to maximise liquidproduction when expanded through an expansion device such as aJoule-Thomson valve is shown in FIG. 1. A typical ideal cooling streammust similarly undergo an enthalpy change throughout the process asshown by the profile in FIG. 2, marked ‘No Cold Recycle’. The secondprofile in FIG. 2 demonstrates the dramatic change in required cooling(i.e. relative change of enthalpy) when large quantities of cold recycleare introduced into the system, marked ‘Cold Recycle’. FIG. 2 showsquantities of cold recycle in the region of 250 kJ/kg (defined ascooling enthalpy per kg of liquid product delivered), which isconsistent with levels of cold recycle used in a fully integratedcryogenic energy system such as the one disclosed in W02007-096656A1. Asis evident from FIG. 2, the addition of the cold recycle completelysatisfies the cooling requirements in the higher temperature end of theprocess.

This presents a problem with current state of the art liquefactionprocesses which are designed to be used with more progressive thermalenergy profiles, and are much more effectively handled by a singlecooling stream running the extent of the heat exchanger. As can be seenfrom FIG. 3 the effective cooling stream produced by current state ofthe art processes (indicated by profile marked ‘state of the art’), suchas the Claude cycle shown in FIG. 4, is extremely linear in comparisonto the required profile in a system using large quantities of coldrecycle (indicated by profile marked ‘Ideal Profile’), and a very poormatch. To meet the acute cooling demand at the lower temperature end, atypical state of the art process must expand a similar quantity of airthrough the cold turbine as a system without cold recycle. This resultsin poor efficiencies and heat transfer requirements above the maximumdesign level of the device within the process heat exchangers.

The present inventors have identified that there is a need for a systemthat can provide focused non-progressive cooling to concentrated areasof the process, in particular at the lower temperature end of theprocess.

SUMMARY OF THE INVENTION

A first aspect of the present invention addresses these needs byproviding, in a first embodiment, a cryogenic liquefaction devicecomprising:

-   -   a heat exchanger;    -   a first phase separator;    -   a first expansion device;    -   a first expansion turbine;    -   a second expansion turbine;    -   a cold recovery circuit including a heat transfer fluid; and    -   an arrangement of conduits, wherein:        -   the operating inlet pressures of the first and second            expansion turbines are different from one another; and        -   the arrangement of conduits is arranged such that:            -   a first portion of a pressurised stream of gas is                directed through the heat exchanger, the first expansion                device and the first phase separator;            -   a second portion of the pressurised stream of gas is                directed through the first expansion turbine, then                through the heat exchanger in a counter-flow direction                to the first portion of the pressurised stream of gas,                and then through the second expansion turbine, and            -   the heat transfer fluid is directed through the heat                exchanger.

In the context of the present invention, the phrase “a counter-flowdirection” is used to mean that the second portion of the pressurisedstream of gas flows through the heat exchanger in an opposite directionto the first portion of the pressurised stream of gas, for at least apart of its path through the heat exchanger. The first and secondportions of the pressurised stream of gas may enter the heat exchangerat opposite ends, i.e. so that the temperature difference between theentry points is maximised. Alternatively, the first and or secondportion of the pressurised stream of gas may enter the heat exchanger ata point between the ends of the heat exchanger, but flow through theheat exchanger in an opposite direction to the other of the first andsecond portion of the pressurised stream of gas, for at least a part ofits path through the heat exchanger.

The cold recovery circuit comprises a thermal energy storage device, ameans for circulating the heat transfer fluid (HTF), and an arrangementof conduits arranged to direct the HTF through the thermal energystorage device and into the heat exchanger. An exemplary cold recoverycircuit is described in detail in GB 1115336.8. The HTF may comprise agas or a liquid, at high or low pressure.

The configuration of the present invention is such that the secondportion of the cooled process stream can be partially expanded throughthe first turbine to provide a high pressure cooling stream local to theentry point of the cold recovery stream of the cold recovery circuit.The stream can then be further expanded through the second turbine toprovide significant additional cooling to the lower section of theprocess.

The present invention offers increased work output from the expansionturbines as a result of the reheating of the expanded stream, whilstalso providing cooling between expansion turbines.

The pressurised stream of gas may consist of gaseous air. Alternatively,the pressurised stream of gas may consist of gaseous nitrogen. Thepressurised stream of gas may be input into the cryogenic liquefactiondevice at a pressure greater than or equal to the critical pressurewhich, for gaseous air is 38 bar and for gaseous nitrogen is 34 bar.

After the pressurised stream of gas is split into two portions, thefirst portion of the pressurised stream of gas and the second portion ofthe pressurised stream of gas may be at the same pressure.Alternatively, the first portion of the pressurised stream of gas andthe second portion of the pressurised stream of gas may be at differentpressures. In particular, the first portion may be above the criticalpressure, and the second portion may be below the critical pressure, orvice versa.

The cryogenic liquefaction device may comprise more than two expansionturbines, with turbines in both parallel and series.

The cryogenic liquefaction device may further comprise a third expansionturbine, wherein the operating inlet pressure of the third expansionturbine is different to at least one of the first and second expansionturbines.

The arrangement of conduits may be such that the third expansion turbineis in parallel with at least one of the first and second turbines suchthat at least a portion of the second portion of the pressurised streamof process gas is directed through the third turbine.

The arrangement of conduits may be such that the third expansion turbineis in series with at least one of the first and second turbines suchthat at least a portion of the second portion of the pressurised streamof process gas is directed through the third turbine.

The cryogenic liquefaction device may further comprise a refrigerantcircuit which is connected to the output of the second expansion turbinevia the arrangement of conduits.

The cryogenic liquefaction device may further comprise a secondarrangement of conduits that directs a second heat transfer fluidthrough a closed cycle refrigeration circuit and through a localisedarea of the heat exchanger. The second heat transfer fluid within therefrigerant circuit may comprise a gas or a liquid, at high or lowpressure.

The cryogenic liquefaction device may further comprise a fourthexpansion turbine, wherein the arrangement of conduits is arranged suchthat:

a third portion of the pressurised stream of gas is directed through thefourth expansion turbine, then through the heat exchanger in acounter-flow direction to the first portion of the pressurised stream ofgas.

The cryogenic liquefaction device may further comprise a fifth expansionturbine, wherein the arrangement of conduits is arranged such that:

the third portion of the pressurised stream of gas is directed throughthe fifth expansion turbine after passing through the fourth expansionturbine and the heat exchanger.

The cryogenic liquefaction device may further comprise a second phaseseparator and a second expansion device, wherein the arrangement ofconduits is arranged such that at least a portion of the second portionof the pressurised stream of gas is directed through the secondexpansion device and the second phase separator after having passedthrough the first expansion turbine.

The or each expansion device may comprise a Joule-Thomson valve, anotherpressure reducing valve, an expansion turbine or another work extractingdevice.

The cryogenic liquefaction device may further comprise a firstcompressor, wherein the arrangement of conduits is arranged such that atleast a portion of the second portion of the pressurised stream of gasis directed through the first compressor before passing through thefirst expansion turbine.

The cryogenic liquefaction device may further comprise a secondcompressor, wherein the arrangement of conduits is arranged such thatthe first portion of the pressurised stream of gas is directed throughthe second compressor before passing through the heat exchanger.

The cryogenic liquefaction device may further comprise a cooler, whereinthe arrangement of conduits is arranged such that the first portion ofthe pressurised stream of gas is directed through the cooler afterpassing through the second compressor and before passing through theheat exchanger.

The cryogenic liquefaction device may further comprise a feed streamcompressor adapted to output the pressurized stream of gas, wherein thearrangement of conduits is arranged firstly such that a feed stream isdirected to the input of the feed stream compressor; and secondly suchthat the output stream from the first phase separator joins the feedstream after passing through the heat exchanger.

The arrangement of conduits may be arranged such that the pressurizedstream of gas output from the feed stream compressor is directed to aheat storage device before passing through the heat exchanger.

The arrangement of conduits may be arranged such that the pressurizedstream of gas output from the heat storage device is directed to a heatrejection device before passing through the heat exchanger.

The first aspect of the present invention further provides a method forthermally balancing a liquefaction process with the use of cold recyclefrom an external thermal energy source comprising:

directing a first portion of a pressurised stream of gas through a heatexchanger, a first expansion device, and a first phase separator;

directing a second portion of a pressurised stream of gas through afirst expansion turbine, then through the heat exchanger in acounter-flow direction to the first portion of the pressurised stream ofgas, and then through a second expansion turbine; and

directing a heat transfer fluid through a cold recovery circuit and theheat exchanger; wherein:

-   -   the operating inlet pressures of the first and second expansion        turbines are different from one another.

Any of the optional features recited above in connection with thecryogenic liquefaction device may also be incorporated into the methodof the first aspect of the present invention.

A second aspect of the present invention addresses these needs byproviding, in a first embodiment, a cryogenic liquefaction devicecomprising: a heat exchanger;

-   -   a first phase separator;    -   a first expansion device;    -   a first expansion turbine;    -   a first compressor;        -   a cold recovery circuit including a heat transfer fluid; and            an arrangement of conduits, arranged such that:            -   a first portion of a pressurised stream of gas is                directed through the heat exchanger, the first expansion                device and the first phase separator;            -   a second portion of the pressurised stream of gas is                directed through the first expansion turbine, then                through the heat exchanger in a counter-flow direction                to the first portion of the pressurised stream of gas,                and then through the first compressor, and            -   the heat transfer fluid is directed through the heat                exchanger.

The arrangement of conduits may be arranged such that the output streamof the first compressor joins the pressurised stream of gas.

The arrangement of conduits may be arranged such that the output streamof the first compressor is directed into the heat exchanger before itjoins the pressurised stream of gas. Alternatively, the arrangement ofconduits may be arranged such that the output stream of the firstcompressor joins the pressurised stream of gas before it is directedinto the heat exchanger.

The first compressor may be either a single stage or a multistagecompressor.

The second aspect of the present invention further provides a method forbalancing a liquefaction process with the use of cold recycle from anexternal thermal energy source comprising:

directing a first portion of a pressurised stream of gas through a heatexchanger, a first expansion device, and a first phase separator;

directing a second portion of a pressurised stream of gas through afirst expansion turbine, then through the heat exchanger in acounter-flow direction to the first portion of the pressurised stream ofgas, and then through a first compressor; and

directing a heat transfer fluid through a cold recovery circuit and theheat exchanger.

Any of the optional features recited above in connection with thecryogenic liquefaction device may also be incorporated into the methodof the second aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described withreference to the figures in which:

FIG. 1 shows a profile of the relative change in total enthalpy which aprocess gas undergoes during the cooling process (Relative Change ofTotal Enthalpy vs Cooled Stream Temperature);

FIG. 2 shows profiles of the relative change in total enthalpy which thecooling streams must undergo during the cooling process for systems withand without the use of large quantities of cold recycle (Relative Changeof Total Enthalpy vs Cooled Stream Temperature);

FIG. 3 shows profiles of the relative change in total enthalpy which thecooling streams must undergo during the cooling process for ‘ideal’,‘state of art’ and ‘present invention’ systems with the use of largequantities of cold recycle (Relative Change of Total Enthalpy vs CooledStream Temperature);

FIG. 4 shows a typical state of the art air liquefaction plantarrangement using the Claude cycle;

FIG. 5 shows a schematic of a cryogenic energy storage systemliquefaction process according to a first embodiment of the first aspectof the present invention;

FIG. 6 shows a second embodiment of the first aspect of the presentinvention;

FIG. 7 shows a third embodiment of the first aspect of the presentinvention;

FIG. 8 shows a fourth embodiment of the first aspect of the presentinvention;

FIG. 9 shows a fifth embodiment of the first aspect of the presentinvention;

FIG. 10 shows a sixth embodiment of the first aspect of the presentinvention;

FIG. 11 shows a seventh embodiment of the first aspect of the presentinvention;

FIG. 12 shows a variation of the second embodiment of the presentinvention;

FIG. 13 shows a variation of the seventh embodiment of the presentinvention;

FIG. 14 shows an eighth embodiment of the present invention;

FIG. 15 shows a variation of the first embodiment of the presentinvention;

FIG. 16 shows another variation of the first embodiment of the presentinvention;

FIG. 17 shows yet another variation of the first embodiment of thepresent invention; and

FIG. 18 shows a first embodiment of the second aspect of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The first simplified embodiment of the present invention is shown inFIG. 5. The system in FIG. 5 is similar to that of the state of the artshown in FIG. 4 in that the main process stream 31 is cooled via coldexpanded air from turbines and expanded through an expansion device suchas a Joule-Thomson Valve 1 to produce liquid, however the warm turbine 3of FIG. 4 is replaced by a second cold turbine 6 aligned in series withthe first cold turbine 5.

In the first embodiment of the present invention shown in FIG. 5, theprocess gas (in the preferred embodiment air) is compressed to highpressure, of at least the critical pressure (which for air is 38 bar,more preferably >45 bar), and at ambient temperature (≈298K) enters thecryogenic liquefaction device at inlet 31, from where it is directedthrough a heat exchanger 100 and cooled progressively by cold lowpressure process gas, before returning through the heat exchanger 100via passage 41, 42. Also passing through the heat exchanger 100 is acold recovery stream 30, 50 of a cold recovery circuit of the cryogenicliquefaction device. The cold recovery circuit comprises: a thermalenergy storage device (not shown); a means for circulating a heattransfer fluid through the cold recovery circuit (not shown); and anarrangement of conduits arranged to direct the heat transfer fluidthrough the thermal energy storage device and the heat exchanger 100. Anexemplary cold recovery circuit is described in detail in GB 1115336.8

A proportion of the high pressure process gas input into the heatexchanger at 31, and now at a temperature of between 150-170K (in thepreferred case 165K), is separated from the main flow 31, via passage39, and is partially expanded to between 5 and 20 bar (more typically10-14 bar), using expansion turbine 5, before passing through passage40, 43 of the heat exchanger 100, where cold thermal energy istransferred to the high pressure gas in stream 35. This feature of thepresent invention provides more effective cooling than the arrangementof FIG. 4 as a result of the higher pressure cooling stream 40, 43around the entry point of the cold recovery stream 30, better matchingthe resultant cooling demand (as shown in FIG. 3) than conventionallayouts, where the warm turbine 3 (of FIG. 4) provides cooling at highertemperatures which are not required where cold recycle is available.

The partially expanded gas stream in passage 40, 43 is heated to atemperature between 120-140K (in the preferred case 125K), as a resultof the thermal transfer in passage 40, 43 through heat exchanger 100,and is further expanded through turbine 6, to between ambient and 6 barwhere it travels through passage 44 and enters the phase separatorvessel 2. The gas fractions of streams 32 and 44 are combined to formoutput stream 34, which travels through passage 41, 42 through heatexchanger 100 which provides additional cooling to the high pressureprocess stream 35. An additional advantage of the present invention isthat the typical composition of the cold process stream in stream 44 isa mixture of liquid and gaseous air. The liquid fraction from the finalexpansion is collected within the phase separator 2 and output viapassage 33.

The numbered points in FIG. 5 indicate positions in the system at whichtypical absolute pressures, temperatures and mass flows are as follows:

Temperature Pressure Mass Flow Point (K) (bar) (kg/hr) 31 298 45 1665135 165.5 45 7160 38 101 45 7160 32 91.23 4 7160 33 91.23 4 6249 34 91.234 911 39 165.5 45 9491 40 113 11.23 9491 43 125.5 11.23 9491 44 95.91 49491 41 95.49 4 10402 42 295.3 4 10402 30 115 1.2 8280 50 295.3 1.2 8280

A second embodiment of the current invention is shown in FIG. 6 (wherelike reference numerals refer to the same components as in FIG. 5),wherein the proportion of air separated from the main stream 31 viapassage 39 is carried out later in the process and therefore at a lowertemperature (between 130-170K). As a result the subsequent temperatureof the cold gas after partial expansion in turbine 5, is sufficient toprovide a high pressure cooling stream for the bottom end of the processstream 35 via passage 40, 43, after which it is expanded again throughthe second turbine 6 to provide additional focussed cooling in stream34.

A third embodiment of the present invention is shown in FIG. 7 (wherelike reference numerals refer to the same components as in FIG. 5)wherein a third expansion turbine 7 is provided in parallel with thesecond turbine 6 which remains in series with turbine 5. Similar to thesecond embodiment shown in FIG. 6, a portion 39 of the cold highpressure stream 31 is partially expanded by turbine 5 to provide a highpressure cooling stream 40 at the lower end of the heat exchanger only,before it is split again into two streams 43, 45 and expanded throughthe two further turbines 6 and 7 in parallel. The outlet from turbine 7is introduced typically into the phase separator 2 via passage 80. Insome embodiments where the cold recycle temperatures are low the outletfrom turbine 7 may be introduced higher up the heat exchanger 100 viapassage 46.

FIG. 8 (where like reference numerals refer to the same components as inFIG. 7) details a fourth embodiment of the present invention wherein,similar to the system shown in FIG. 7, a third expansion turbine 7 isadded and placed in parallel to the second expansion turbine 6 whichremains in series with the first expansion turbine 5. The expansionratios of the second 6 and third 7 turbines are different from eachother, the second expanding from around 8 bar to 4.5 bar, and the thirdexpanding from around 8 bar to near ambient. The inventors have realisedthat by layering multiple cooling streams in parallel as in FIG. 8, thecooling profile demand, identified in FIG. 3, can be more closelymatched. In some embodiments, where the outlet pressure of turbine 7 issubstantially equal to the separator pressure 2, the outlet of turbine 7is introduced to the phase separator 2 via passage 80 where liquidformed in the outlet of turbine 7 is collected.

A further embodiment is shown in FIG. 9 (where like reference numeralsrefer to the same components as in FIG. 8). This embodiment is the sameas that of FIG. 8 except that the exiting gases travelling throughstream 48 from the second expansion turbine 6 are removed from theprocess heat exchanger 100 before reaching the top. The cold gases instream 48 are further compressed, by compressor 8, and the resultantstream 49 is cooled by a closed cycle refrigeration circuit 10 beforeexiting the circuit 10 as stream 51 and mixing with the high pressureprocess stream 31. In certain embodiments there is the potential for aproportion of liquid to be formed in stream 46 from the cold gas onexiting the third turbine 7, whereby the stream would be directed viapassage 80 to enter the phase separator 2, instead of being directedstraight through the heat exchanger 100 via passageway 46, 47.

In a further embodiment (not shown but otherwise the same as FIG. 9),the outlet of turbine 7 may be expanded to near ambient so that thisprocess stream can be used to drive a low pressure high grade coldstore, such as that detailed in GB1115336.8.

The embodiment shown in FIG. 10 (where like reference numerals refer tothe same components as in FIG. 5) is the same as that of FIG. 5 exceptfor the addition of a closed cycle refrigeration circuit 101 to providea local potentially high pressure cooling stream 60 to better match thecooling demand. The closed cycle refrigeration circuit 101 includescompressor 102, cooler 103 and expansion turbine 104.

FIG. 11 shows a further embodiment of the present invention (where likereference numerals refer to the same components as in FIG. 5) wherein awarm turbine 14 and cold turbine 5 partially expand portions 60, 39 ofthe cold high pressure stream 31. Streams 60 and 39 are at differenttemperatures and are expanded to different pressures by turbines 14 and5 to provide streams 61 and 40, respectively. Gas in streams 61 and 40provides focussed cooling to the high pressure stream at points 35 and69, before separately being expanded to between 0 and 6 bar, usingfurther turbines 16 and 6 to provide streams 63 and 44 which aredirected through heat exchanger 100.

A variation to the second embodiment is shown in FIG. 12 (where likereference numerals refer to the same components as in FIG. 6) whereinthe addition of a second phase separator 18 and pressure reducing valve19 enable the removal of additional liquid produced in stream 40. Insome embodiments the outlet pressure of turbine 6 is equal to theseparator pressure 2 and the outlet of turbine 6 is introduced to thephase separator via passage 80 where liquid formed in the outlet toturbine 7 is collected.

A further component (not shown), which can be included in any of theprevious embodiments is a closed loop refrigeration cycle (similar tocycle 101 shown in FIG. 10), that utilises a different working fluid toprovide additional cooling at a specific section of the system where thecooling requirements are particularly high, in particular between 140and 120K. The different working fluid may comprise a refrigerant such asmethane.

A further arrangement, which can be applied to any of the previousembodiments where the high pressure stream is divided into two streamsof different pressure, includes providing the first stream (that iscooled and then transferred to the expansion device) at a pressure abovethe critical pressure to maximise liquid production. The second highpressure stream is at a different pressure (typically above the firststream pressure) and is cooled and transferred to the two or moreexpansion turbines to provide additional cooling to the first stream asdescribed in the previous embodiments.

In a further embodiment as shown in FIG. 13 (where like referencenumerals refer to the same components as in FIG. 5) the second stream 58is compressed by compressor 20 to stream 59 and is then divided into afurther two or more streams 63, 65. Stream 65 is compressed bycompressor 19 and then directed, via a first stream (66), through twoturbines 5, 6 in series. Stream 63 is expanded through a third turbine21. The outlet streams 40, 44, 64 of the first, second and thirdturbines 5, 6, 21 provide additional cooling for the first processstream 35 prior to expansion in an expansion device such as aJoule-Thomson valve 1.

In a further embodiment, as shown in FIG. 14 (where like referencenumerals refer to the same components as in FIG. 5) applied to the firstembodiment, the cooled gas stream 31 is fed directly from a compressorcommonly referred to as a Recycle Air Compressor (RAC) and a stream 58is split from the cooled gas stream 31 and subsequently boosted to ahigher pressure by compressor 19 before being directed through expansionturbines 5 and 6 and heat exchanger 100. This additional boostercomponent can be incorporated into any of the previous embodiments.

FIG. 15 (where like reference numerals refer to the same components asin FIG. 14) shows a variation of the embodiment of FIG. 16 wherebystream 31 is fed directly from the RAC. Stream 31 is split into twostreams 41 and 35; stream 41 is directed through heat exchanger 100,where it is cooled before being directed through expansion turbines 5and 6 and again heat exchanger 100, while stream 35 is boosted to ahigher pressure by booster 19 before being directed through heatexchanger 100 and an expansion device such as a Joule-Thomson valve 1.

FIG. 16 (where like reference numerals refer to the same components asin FIG. 15), shows a variation where stream 31 is again fed directlyfrom the RAC and compressed to a pressure lower than the criticalpressure (<38 bar). Stream 41 splits from the main cooled stream 31prior to the remainder of the main cooled stream 35 being boosted andsubsequently cooled by boosters 19 and 20, and coolers 10 and 22. Thesub critical pressure stream 41 is cooled via heat exchanger 100 beforebeing partially expanded, to between 5 and 20 bar, but more typically10-14 bar, through expansion turbine 5 before passing through passage40, 43, of the heat exchanger 100, where cold thermal energy istransferred to the high pressure gas in stream in passage 73, 38, andbeing further expanded by expansion turbine 6. The additional componentsarranged in stream 35 can also be incorporated in any of the previousembodiments.

In the final embodiment, as shown in FIG. 17 (where like referencenumerals refer to the same components as in FIG. 15) shows a variationwhere output stream of the first phase separator 2 becomes low pressurereturn vapour 42, having passed through the heat exchanger 100, andmerges with a feed stream 401 to form stream 402. The pressure of stream402 can be between 3 barA and 15 barA, more typically 8 barA. Stream 402is directed to a single stage compressor 400, which boosts the stream402 to a higher pressure. The output stream 403 of the single stagecompressor 400 therefore has a higher pressure than stream 402. Thehigher pressure is at least the critical pressure (which for air is 38bar, more preferably >45 bar). The temperature of stream 403 can bebetween 100 deg C and 400 deg C, more typically 270 deg C. Stream 403 isdirected to a heat storage device 404 which removes at least some of theheat energy in the stream 403. The temperature of the output stream 405of the heat storage device 404 can be between 20 deg C and 100 deg C,more typically 60 deg C. If the temperature of stream 405 is aboveambient temperature the heat rejection device 406 may be used to coolthe temperature of the stream. When this liquefaction cycle is used aspart of a cryogenic energy storage plant it is highly preferable thatthe heat of compression captured by the heat storage device 404 is usedin the power recovery cycle to boost the temperature of the workingfluid at the inlet of the expansion turbines.

FIG. 18 (where like reference numerals refer to the same components asin FIG. 4) shows an embodiment of the invention which is a furtherdevelopment of the air liquefaction plant arrangement of FIG. 4. Here,the cold vapour stream 40, which is output from the expansion turbine 4,is directed to the heat exchanger 100 rather than merging with returningstream 34 to form stream 41 as shown in FIG. 4. The cold vapour stream40 thus gains heat as it passes through the heat exchanger 100 and exitsthe heat exchanger as stream 43. The temperature of stream 43 can bebetween 0 deg C and −180 deg C, more typically −117 deg C. Stream 43 isdirected to a compressor 300, which boosts the stream 43 to a higherpressure. The compressor 300 can be a multistage compressor or a singlestage compressor. The output stream 301 of the compressor 300 isdirected back to heat exchanger 100 in one of two arrangements. If thetemperature of stream 301 is near ambient temperature then it can bedirected to merge with stream 35 outside the heat exchanger 100. This isshown by stream 302. Alternatively, if the temperature of stream 301 isbelow ambient temperature then it can be directed to merge with thestream 35 insider the heat exchanger to form stream 74. This is shown bystream 303.

It will of course be understood that the present invention has beendescribed by way of example, and that modifications of detail can bemade within the scope of the invention as defined by the followingclaims.

1. A cryogenic liquefaction device comprising: a heat exchanger; a firstphase separator; a first expansion device; a first expansion turbine; asecond expansion turbine; a cold recovery circuit including a heattransfer fluid; and an arrangement of conduits, wherein: the operatinginlet pressures of the first and second expansion turbines are differentfrom one another; and the arrangement of conduits is arranged such that:a first portion of a pressurised stream of gas is directed through theheat exchanger, the first expansion device and the first phaseseparator; a second portion of the pressurised stream of gas is directedthrough the first expansion turbine, then through the heat exchanger ina counter-flow direction to the first portion of the pressurised streamof gas, and then through the second expansion turbine, and the heattransfer fluid is directed through the heat exchanger.
 2. The cryogenicliquefaction device of claim 1 wherein the cold recovery circuitcomprises: a thermal energy storage device; a means for circulating theheat transfer fluid; and an arrangement of conduits arranged to directthe heat transfer fluid through the thermal energy storage device andthe heat exchanger.
 3. The cryogenic liquefaction device of claim 1,wherein the pressurised stream of gas consists of gaseous air or gaseousnitrogen.
 4. The cryogenic liquefaction device of claim 3, wherein thepressurised stream of gas is input into the cryogenic liquefactiondevice at a pressure greater than or equal to the critical pressure. 5.The cryogenic liquefaction device of claim 1, wherein the first portionof the pressurised stream of gas and the second portion of thepressurised stream of gas are at different pressures.
 6. The cryogenicliquefaction device of claim 1, wherein the first portion of thepressurised stream of gas and the second portion of the pressurisedstream of gas are at the same pressure.
 7. The cryogenic liquefactiondevice of claim 1, and further comprising a third expansion turbine,wherein: the operating inlet pressure of the third expansion turbine isdifferent to at least one of the first and second expansion turbines. 8.The cryogenic liquefaction device of claim 7, wherein the arrangement ofconduits is such that the third expansion turbine is in parallel with atleast one of the first and second turbines such that at least a portionof the second portion of the pressurised stream of process gas isdirected through the third turbine.
 9. The cryogenic liquefaction deviceof claim 7, wherein the arrangement of conduits is such that the thirdexpansion turbine is in series with at least one of the first and secondturbines such that at least a portion of the second portion of thepressurised stream of process gas is directed through the third turbine.10. The cryogenic liquefaction device of claim 1, and further comprisinga refrigerant circuit which is connected to an output of the secondexpansion turbine via the arrangement of conduits.
 11. The cryogenicliquefaction device of claim 1, and further comprising a secondarrangement of conduits that directs a second heat transfer fluidthrough a closed cycle refrigeration circuit and through a localisedarea of the heat exchanger.
 12. The cryogenic liquefaction device ofclaim 11, wherein the second heat transfer fluid comprises a gas or aliquid.
 13. The cryogenic liquefaction device of claim 7, and furthercomprising a fourth expansion turbine, wherein: the arrangement ofconduits is arranged such that: a third portion of the pressurisedstream of gas is directed through the fourth expansion turbine, and thenthrough the heat exchanger in a counter-flow direction to the firstportion of the pressurised stream of gas.
 14. The cryogenic liquefactiondevice of claim 13, and further comprising a fifth expansion turbine,wherein: the arrangement of conduits is arranged such that: the thirdportion of the pressurised stream of gas is directed through the fifthexpansion turbine after passing through the fourth expansion turbine andthe heat exchanger.
 15. The cryogenic liquefaction device of claim 1,wherein the first expansion device comprises at least one of aJoule-Thomson valve, another pressure reducing valve, an expansionturbine and another work extracting device.
 16. The cryogenicliquefaction device of claim 1, and further comprising a second phaseseparator and a second expansion device, wherein the arrangement ofconduits is arranged such that at least a portion of the second portionof the pressurised stream of gas is directed through the secondexpansion device and the second phase separator after having passedthrough the first expansion turbine.
 17. The cryogenic liquefactiondevice of claim 16, wherein the second expansion device comprises atleast one of a Joule-Thomson valve, another pressure reducing valve, anexpansion turbine and another work extracting device.
 18. The cryogenicliquefaction device of claim 1, and further comprising a firstcompressor, wherein the arrangement of conduits is arranged such that atleast a portion of the second portion of the pressurised stream of gasis directed through the first compressor before passing through thefirst expansion turbine.
 19. The cryogenic liquefaction device of claim18, and further comprising a second compressor, wherein the arrangementof conduits is arranged such that the first portion of the pressurisedstream of gas is directed through the second compressor before passingthrough the heat exchanger.
 20. The cryogenic liquefaction device ofclaim 19, and further comprising a cooler, wherein the arrangement ofconduits is arranged such that the first portion of the pressurisedstream of gas is directed through the cooler after passing through thesecond compressor and before passing through the heat exchanger.
 21. Thecryogenic liquefaction device of claim 1, wherein the output from thesecond expansion turbine is directed into the first phase separator. 22.The cryogenic liquefaction device of claim 7, wherein an output from thethird expansion turbine is directed into the first phase separator. 23.The cryogenic liquefaction device of claim 13, wherein an output fromthe fourth expansion turbine is directed into the first phase separator.24. The cryogenic liquefaction device of claim 14, wherein an outputfrom the fifth expansion turbine is directed into the first phaseseparator.
 25. The cryogenic liquefaction device of claim 1, furthercomprising a feed stream compressor adapted to output the pressurizedstream of gas, wherein the arrangement of conduits is arranged suchthat: a) a feed stream is directed to an input of the feed streamcompressor; and b) an output stream from the first phase separator joinsthe feed stream after passing through the heat exchanger.
 26. Thecryogenic liquefaction device of claim 25, wherein the arrangement ofconduits is arranged such that the pressurized stream of gas output fromthe feed stream compressor is directed to a heat storage device beforepassing through the heat exchanger.
 27. The cryogenic liquefactiondevice of claim 26, wherein the arrangement of conduits is arranged suchthat a pressurized stream of gas output from the heat storage device isdirected to a heat rejection device before passing through the heatexchanger.
 28. A cryogenic liquefaction device comprising: a heatexchanger; a first phase separator; a first expansion device; a firstexpansion turbine; a first compressor; a cold recovery circuit includinga heat transfer fluid; and an arrangement of conduits, arranged suchthat: a first portion of a pressurised stream of gas is directed throughthe heat exchanger, the first expansion device and the first phaseseparator; a second portion of the pressurised stream of gas is directedthrough the first expansion turbine, then through the heat exchanger ina counter-flow direction to the first portion of the pressurised streamof gas, and then through the first compressor, and the heat transferfluid is directed through the heat exchanger.
 29. The cryogenicliquefaction device of claim 28, wherein the arrangement of conduits isarranged such that an output stream of the first compressor joins thepressurised stream of gas.
 30. The cryogenic liquefaction device ofclaim 29, wherein the arrangement of conduits is arranged such that theoutput stream of the first compressor is directed into the heatexchanger before it joins the pressurised stream of gas.
 31. Thecryogenic liquefaction device of claim 29, wherein the arrangement ofconduits is arranged such that the output stream of the first compressorjoins the pressurised stream of gas before it is directed into the heatexchanger.
 32. The cryogenic liquefaction device of claim 28, whereinthe first compressor is one of a single stage and a multistagecompressor.
 33. A cryogenic energy storage device including thecryogenic liquefaction device claim
 1. 34. A method for balancing aliquefaction process with the use of cold recycle from an externalthermal energy source comprising: directing a first portion of apressurised stream of gas through a heat exchanger, a first expansiondevice, and a first phase separator; directing a second portion of apressurised stream of gas through a first expansion turbine, thenthrough the heat exchanger in a counter-flow direction to the firstportion of the pressurised stream of gas, and then through a secondexpansion turbine; and directing a heat transfer fluid through a coldrecovery circuit and the heat exchanger; wherein: operating inletpressures of the first and second expansion turbines are different fromone another.
 35. A method for balancing a liquefaction process with theuse of cold recycle from an external thermal energy source comprising:directing a first portion of a pressurised stream of gas through a heatexchanger, a first expansion device, and a first phase separator;directing a second portion of a pressurised stream of gas through afirst expansion turbine, then through the heat exchanger in acounter-flow direction to the first portion of the pressurised stream ofgas, and then through a first compressor; and directing a heat transferfluid through a cold recovery circuit and the heat exchanger.
 36. Amethod of storing energy including the method for balancing aliquefaction process of claim
 34. 37. A method of storing energyincluding the method for balancing a liquefaction process of claim 35.